1. Field of the Invention
This invention relates to an optical sensor, an optical temperature-measuring device and a measuring method using the optical sensor, which are capable of measuring temperature etc. by detecting Raman scattering light which is generated in an optical fiber etc.
2. Description of the Related Art
An optical sensor is known that measures temperature, distortion, and pressure, or detects breaking portion by using an optical fiber etc. Particularly, an optical temperature sensor using Raman scattering of the optical fiber is known.
FIG. 7 is a circuit diagram showing an optical temperature-measuring device 70 composed by connecting an electric circuit 72 to an optical temperature sensor 71.
As shown in FIG. 7, the optical temperature sensor 71 comprises a sensing portion with an optical fiber (=measurement long-distance optical fiber) 73 disposed at temperature measurement points, a light source (=light-emitting device) 75 to output a light signal to the optical fiber 73, and two photodetectors (=light-receiving devices) 76, 77 to receive a backscattered light from the optical fiber 73. The light source 75 and the light-receiving devices 76, 77 are connected with the one optical fiber 73 through a wavelength filter 74.
The light source 75 and the two light-receiving devices 76, 77 are connected electrically with an electric circuit 72 respectively. For example, received signal amplifiers 78, 78 to amplify a signal from the light-receiving devices 76, 77 are connected with the light-receiving device 76 and 77 respectively, analog-digital converters 79, 79 (A/D converters) are each connected with the received signal amplifiers 78, 78 respectively, and the both A/D converters 79, 79 are connected with a signal processing circuit 80. Further, the light source 75 is connected with the signal processing circuit 80 through a light-emitting device drive circuit 81.
The optical fiber 73 is a multimode fiber or a single-mode fiber etc. for general communication, and its core is doped with Ge.
When light of the light-emitting device 75 such as a laser diode is inputted to the optical fiber 73, a slight Raman scattering light is generated in each point of the optical fiber 73. As shown in FIG. 8, the Raman scattering light is generated at wavelength bands on both sides of incident wavelength λ0. The Raman scattering light on the longer-wavelength side is called Stokes light λSt and the Raman scattering light on the shorter-wavelength side is called anti-Stokes light λAs. The strength ratio of the Stokes light and the anti-Stokes light generated in the optical fiber 73 depends on the temperature of the optical fiber 73. Thus, when the temperature of the optical fiber 73 is changed depending on the temperature of a temperature measurement object, the strength ratio of the detected Stokes light and anti-Stokes light is changed. Therefore, the temperature of the temperature measurement object can be measured by detecting the strength ratio.
In the optical temperature-measuring device 70, backscattered Stokes light and anti-Stokes light are separated by the wavelength filter 74, and are received by light-received device 76 and 77, respectively. The received light is converted into an electric signal, and the electric signal is amplified by the received signal amplifier 78. The amplified electric signal is converted into a digital signal by the A/D converter 79, and is inputted to the signal processing circuit 80. In the signal processing circuit 80, temperature is determined from the input electric signal, and its thermal signal is displayed.
In general, since the Raman scattering light is very weak in its strength, the electric signal converted by the light-receiving devices 76, 77 has a low S/N ratio. Therefore, in order to improve the S/N ratio and the measurement accuracy of temperature, the Raman scattering light is detected many times and the electric signals detected are averaged.
The related art of the invention is, e.g., JP-A-2784199, which discloses an optical temperature sensor using Raman scattering light.
In the optical temperature sensor 71 in FIG. 7, pulsed light is inputted to the optical fiber 73 and the Raman scattering light generated by the pulsed light is detected. In the optical temperature sensor 71 using the pulsed light, axial resolution is determined by the pulse width of the pulsed light or the sampling frequency when converting the received signal.
In the system to receive the Raman scattering light, the axial resolution Δx [m] in the temperature measurement is given by the following formula:Δx=cW/2n, where the pulse width is W [s], the light speed is c [m/s] and the refractive index of the optical fiber is n.
Thus, provided that c=3×108 [m/s] and n=1.5, the pulsed light of 10 ns in pulse width must be inputted into the optical fiber to obtain an axial resolution of 1 m. Further, the pulsed light of 1 ns in pulse width must be inputted thereinto to obtain an axial resolution of 0.1 m.
Further, the axial resolution Δx [m] determined by the sampling frequency is given by the following formula:Δx=c/2nfs, where the sampling frequency is fs [Hz].
Thus, provided that c=3×108 [m/s], n=1.5 and fs=100 [MHz], the axial resolution Δx becomes 1 m. Further, the sampling frequency must be fs=1 [GHz] to obtain an axial resolution of 0.1 m.
Accordingly, the sampling frequency must be at least 1 [GHz] or more to adjust the axial resolution of the optical temperature sensor to be 0.1 m or less, so that a high-speed operation circuit with a pulse width of 1 ns or less in pulsed is required. However, such a circuit is difficult to provide at a low cost based on the present technology level.